1. Field
Embodiments relate generally to control systems and more specifically, yet not exclusively to control of variable-speed direct-driven PMSG (permanent magnet synchronous generator) wind turbines.
2. Background
Wind power is today's most rapidly growing renewable energy source. Large-scale wind generation facilities have become a very visible component of the interconnected power grid in many portions of the United States and around the world. Wind turbines can operate at either fixed speed or variable speed. For a fixed-speed wind turbine, the generator is directly connected to the electrical grid. For a variable speed wind turbine, the generator is controlled by power electronic equipment. The two most-common variable speed wind turbines are wind turbines that use direct-driven synchronous generators (DDSG) or double-fed induction generators (DFIG). For both of them, a frequency converter such as a pulse-width modulation (PWM) AC/DC/AC frequency converter is connected between the grid and the generator. FIG. 1A illustrates an embodiment of an AC/DC/AC converter in modern variable-speed synchronous generator wind turbine, and FIG. 1B illustrates an embodiment of an AC/DC/AC converter in variable-speed DFIG wind turbine. For a DDSG wind turbine, the converter is connected between the generator and the grid, as shown in FIG. 1A, and for the DFIG wind turbine, the converter is connected between the rotor circuit and the grid, as shown in FIG. 1B. Traditionally, each of these two PWM converters is controlled by using decoupled d-q control approaches, as are known in the art. Basically, the machine-side converter controls the real and reactive power production of the electric generator, and the grid-side converter controls the dc-link voltage and the reactive power absorbed from the grid by the converter. The general control technique for the grid-side converter control, which is widely used in wind power industry, is a decoupled d-q control approach that uses the direct (d) axis current component for real power control and quadrature (q) axis current component for reactive power control. By controlling the converters on both sides, characteristics can be adjusted so as to achieve maximum of effective power conversion or capturing capability for a wind turbine and to control its power generation with less fluctuation.
The variable speed wind turbine with a multi-pole permanent magnet synchronous generator (PMSG) and full-scale power converter is considered to be a promising, but not yet very popular wind turbine concept. A multi-pole synchronous generator connected to a power converter can operate at low speeds, so that a gearbox can be omitted. Because a gearbox causes higher weight, losses, costs and maintenance, a gearless construction represents an efficient and robust solution. Moreover, due to the permanent magnet excitation of the generator, the DC excitation system can be eliminated. The efficiency of a PMSG wind turbine is thus assessed to be higher than other variable-speed wind turbine concepts. In addition, a full scale insulated-gate bipolar transistor (IGBT) back-to-back voltage source converter, by which the generator is connected to the power grid, allows full controllability of the system. Due to the intensified grid codes such as strong short-circuit ride-through capability, wind turbines with full scale power converters are favored in the future compared to wind turbine concepts using doubly-fed induction generators.
However, the energy captured and converted from the wind by a PMSG wind turbine depends not only on the synchronous generator but also on the integration of aerodynamic, electrical and power converter systems of the wind turbine as well as how they are controlled under variable wind conditions. At present, commercial PMSG technology mainly uses a passive rectifier followed by an IGBT inverter. FIG. 1C illustrates an embodiment of the structure of a PMSG system, which is comprised of a standard permanent magnet synchronous machine with the stator winding connected to the grid through a frequency converter. In modern PMSG wind turbine designs, the frequency converter can also be comprised of two self-commutated PWM converters, machine- and grid-side converters, with an intermediate DC voltage link. The DC-link created by the capacitor in the middle decouples the operation of the two converters, thus allowing their control and operation to be optimized. FIG. 1D illustrates a PMSG wind turbine with full-scale/fully-controllable PWM converters mainly comprising three parts: a wind turbine drive train, a permanent magnet synchronous generator, and a back to back voltage source PWM converter. In the turbine drive train, the rotor blades of a wind turbine catch wind energy that is then transferred to the generator. The generator, converting mechanical energy into electrical energy, is a standard synchronous machine with its stator windings connected to the grid through a frequency converter. The frequency converter is built by two current-regulated voltage-source PWM converters, a machine-side converter (MSC) and a grid-side converter (GSC), with a dc voltage link.
The control in a PMSG system has three levels: the generator level, the wind turbine level, and the central wind power plant level. At the generator level, each of the two PWM converters (FIG. 1D) is controlled through decoupled d-q vector control approaches as known in the existing technology. The MSC controls the PMSG to achieve the following goals: maximum energy extraction from the wind and/or compliance with a grid control demand. The GSC maintains a constant dc-link voltage and adjusts reactive power absorbed from the grid by the converter. At the wind turbine level, there is a speed controller and a power limitation controller. At low wind speed, the speed controller gives a power or torque reference to the MSC controller based on the principle of maximum energy capture. The power limitation controller increases or decreases the pitch angle of wind turbine blades to prevent the turbine from going over the rated power at a high wind speed. At the central wind power plant level, the power production of the entire plant is determined based on the grid requirements. The central control system sends out reference power signals to each individual wind turbine according to a grid need, while the local turbine control system ensures that the reference power signal sent by the central control system is reached. Thus, the performance of a PMSG wind turbine depends not only on the wind but also on how effectively the generator and the turbine aerodynamic system are coordinated under variable wind and complex control conditions. The control objectives of the machine-side converter include 1) maximum energy extraction from the wind, and 2) management of PMSG energy generation in compliance with grid demands. However, many of the approaches taken to control PMSG energy generation results in less than optimal energy generation of the PMSG wind turbine and less than optimal reliability, stability, and power quality of both, the PMSG and electric utility systems.
Therefore, what is desired are control systems and methods that overcome challenges present in the art, some of which are described above.